Various imaging modalities have been used to identify and visualize mineral content of rocks, both in two dimensions (2D) and in three dimensions (3D). For example, these imaging modalities can analyze rock samples from oil and gas extraction operations to determine porosity and mineralogy to model flow and mechanical characteristics of the samples obtained during exploration and production operations. Generally, these imaging modalities are characterized as destructive and non-destructive techniques. Further, some modalities only analyze surface features, whereas others can analyze three-dimensional structure.
In typical operation, these imaging modalities create image datasets such as 3D volumes or 2D images. Image analysis techniques are then employed to infer mineral content from the volumes and the images created by the different imaging modalities.
Non-destructive imaging systems include x-ray computed tomography (CT) microscopy and Scanning Electron Microscopy (SEM) systems. These systems provide the ability to visualize features such as pores, organics and minerals in the samples.
The X-ray CT microscopy systems irradiate the sample with x-rays, typically in a range between 1 and several hundred keV. 2D projection images are collected at multiple angles and a 3D volume of the sample is reconstructed from the projections. While the CT intensity correlates with mineral density, there is no direct way of identifying mineralogy on an X-ray CT microscope system.
SEM systems instead irradiate the sample surface with a beam of high energy electrons, typically between 500 eV and 30 keV. The signals derived from electron-sample interaction are used in constructing high resolution 2D images of the sample surface. This enables the simultaneous operation of SEM in multiple modes such as Back Scattered Electron (BSE), Secondary Electron (SE), Energy Dispersive X-ray (EDX), and Cathodoluminescence (CL) modes. EDX is typically the primary system on a SEM that offers quantitative mineralogy information which enables 2D mineral mapping of the sample surface.
Destructive imaging systems include Focused Ion Beam Scanning Electron Microscope (FIB-SEM) systems. A FIB-SEM is a multiple beam system that integrates ion beam and electron beam systems. FIB system irradiates the sample with a focused high-current beam of ions of a source material such as gallium to mill the sample surface with high precision. The milled surface is then imaged at high resolution using the integrated SEM system. The FIB milling and SEM imaging process is repeated until a desired volume is sampled. The SEM images from each slice are stacked to construct a 3D volume of the milled region of the sample.
One current imaging analysis technique creates a 3D mineral map of the sample by analyzing volume image datasets of a sample created from x-ray imaging systems. A total mineral content of the sample is then defined, and x-ray attenuation coefficients are calculated for the defined minerals. The technique then segments the grey scale 3D images by identifying characteristic grey scale levels in the images corresponding to the calculated x-ray attenuation coefficients.
Another imaging analysis technique employs multi-phase segmentation of 3D x-ray tomography volume image datasets. The 3D x-ray tomography volumes are processed to obtain a standardized intensity grey scale images, which are then segmented into at least 3 phases. The segmentation steps include computing a median/mean-filtered-gradient image of the standardized intensity image, creating an intensity vs. gradient graph from the median/mean-filtered-gradient image and the standardized intensity image, partitioning the intensity vs. gradient graph into at least 3 regions, and using thresholds defining the regions to segment the standardized grey scale image to create the segmented image. Then, volumetric fractions and spatial distributions of the segmented phases are calculated and compared with target values.